There are many methods for treating sewage, which can generally be classified into four categories: biological treatment, physical treatment, chemical treatment, and natural treatment.

1. Biological treatment method

A wastewater treatment method that converts organic pollutants in solution, colloid, and fine suspended state in wastewater into stable and harmless substances through the metabolic action of microorganisms. According to the different microorganisms involved, biological treatment can be divided into two types: aerobic biological treatment and anaerobic biological treatment. The widely used method for wastewater biological treatment is aerobic biological treatment, which can be divided into two categories according to their different process methods: activated sludge method and biofilm method. The activated sludge process itself is a treatment unit with multiple operating modes. The treatment equipment for biofilm method includes biofilters, biological turntables, biological contact oxidation tanks, and biological fluidized beds. Biological oxidation pond method, also known as natural biological treatment method. Anaerobic biological treatment, also known as bioreduction treatment, is mainly used to treat high concentration organic wastewater and sludge.

2. Physical processing method
The methods of separating and recovering insoluble suspended pollutants (including oil film and oil droplets) in wastewater through physical action can be divided into gravity separation, centrifugal separation, and sieve filtration retention. The processing units belonging to the gravity separation method include sedimentation, flotation (air flotation), etc. The corresponding processing equipment used is sedimentation tank, sedimentation tank, oil separation tank, air flotation tank and its ancillary devices, etc; The centrifugal separation method itself is a processing unit, and the processing devices used include centrifugal separators and water cyclone separators; The screening and interception method has two types of treatment units: grid screen interception and filtration. The former uses processing equipment such as grids and sieves, while the latter uses sand filters and microporous filters. The processing method based on the principle of heat exchange also belongs to the physical processing method, and its processing units include evaporation, crystallization, etc.

3. Chemical treatment method
A wastewater treatment method that separates and removes dissolved or colloidal pollutants from wastewater through chemical reactions and mass transfer, or converts them into harmless substances. In chemical treatment methods, the treatment units based on chemical reactions generated by adding chemicals are coagulation, neutralization, oxidation-reduction, etc; The processing units based on mass transfer include extraction, stripping, stripping, adsorption, ion exchange, electrodialysis, and reverse osmosis. The latter two processing units are collectively referred to as membrane separation technology. The processing unit that utilizes mass transfer has both chemical and physical effects related to it, so it can also be separated from chemical processing methods and become another type of processing method, called physical-chemical method.

4. Natural processing method
The principle of using nature (natural water bodies, soil, etc.) to self purify sewage is utilized to exert its effect. Including land treatment systems and aquatic plant treatment systems. Common methods include biological ponds, soil treatment methods, and artificial wetland treatment methods. Oxidation pond is a natural or artificially constructed pond used for biological treatment of wastewater. Wastewater stays in the pond for a long time, while microorganisms in the water can metabolize and degrade organic pollutants. Dissolved oxygen is achieved through the photosynthesis of algae and the reoxygenation of the pond surface, which can greatly reduce organic pollutants in the water and to some extent remove nitrogen and phosphorus from the water, reducing eutrophication of the water.